In WLAN (Wireless Local Area Networks), after a user terminal enters a coverage scope of an AP (Access Point), the access point sends a specific association identifier to the user terminal and may broadcast a beacon frame periodically. Each beacon frame contains a TIM (Traffic Indication Map), and a bitmap in the TIM may indicate whether data needing to be transmitted to the user terminal are cached in the access point. After receiving the beacon frame, the user terminal may read the TIM in the beacon frame by the specific association identifier so as to know whether there are data cached in the access point. If the user terminal knows there are data cached in the access point, the user terminal may wait for receiving the data transmitted by the access point, and if the user terminal reads there is no data cached in the access point, the user terminal may enter a dormant state to save power.
However, with regard to the prior art, the inventor has found that if the quantity of the user terminals with cached data in the access point is close to the quantity of user terminals without cached data, time for generating the TIM in the beacon frame by the access point is still very long, that is, the quantity of bytes occupied by the TIM is still large, and lots of time is needed for transmitting the beacon frame. Moreover, association identifiers of user terminals are arrayed according to a certain sequence, if the distances among the association identifiers of a plurality of user terminals are far, the time for generating the TIM in the beacon frame by the access point is also very long, that is, the quantity of bytes occupied by the TIM is large, and then lots of time is also needed for transmitting the beacon frame.